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Community and Q&A

‘Rim joist’ frost and foam insulation

rshuman | Posted in Energy Efficiency and Durability on

The ‘rim joist’ in my basement was insulated with pieces of roxul placed in stud bays. I wish to seal this area with a couple inches of spray foam and then reinstall the roxul.

Along the gable ends of the house, a sill plate was placed at the top of the concrete foundation. Short studs extend upward from the plate about 8″ to a double plate. Exterior sheathing spans the height between the sill plate and the double plate above, rather than a rim joist made of 2-by stock.

When I removed the roxul from the stud along the gable side of the house I noticed very light frost in some of them. I am wondering if I should be overly concerned about the frost and/or reconsider my plan to spray a couple inches of foam along the rim. If I proceed with spraying, I assume I should let the frosted areas completely dry before proceeding. Correct?


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  1. Expert Member
    BILL WICHERS | | #1

    The frost is the result of moisture migrating through the permeable roxul and coming into contact with the below freezing inside surface of the rim joist. This is a common problem when using air permeable insulation to insulate the rim joist.

    Spray foam won’t have this problem (I’m assuming you’ll be using closed cell spray foam), since it will act as a vapor barrier/retarder, depending on thickness. You do want the rim joist to dry out before applying the spray foam. You also want it to warm up first — spray foam doesn’t work as well on very cold surfaces. Wait until things warm up in the late spring.


  2. Peter Yost | | #2

    Hi Rob -

    Interesting that this happened along one wall and not the others; for some reason this section of wall gets colder. It may have more to do with which direction this gable wall faces than anything else (perhaps that is the predominant-wind side of the building). But might be a good idea to determine WHY this wall section had frost compared to the others, since it could affect the solution.


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